An organic electroluminescence (organic EL) device is an organic light-emitting diode (OLED) in which the light emitting layer is a film made from organic compound that emits light in response to an electric current. The organic EL device is applied to flat panel displays due to its high illumination, low weight, ultra-thin profile, self-illumination without back light, low power consumption, wide viewing angle, high contrast, simple fabrication methods and rapid response time.
The first diode device was reported by Ching W. Tang and Steven Van Slyke at Eastman Kodak in 1987. The device used a two-layer structure with separate hole transporting and electron transporting layers such that recombination and light emission occurred in the middle of the organic layer. This resulted in reduction of operating voltage and improvement of the efficiency, thereby leading to the current area of organic EL device research and device production.
Typically, the organic EL device is composed of organic material layers sandwiched between two electrodes. The organic material layers include the hole transporting layer, the light emitting layer, and the electron transporting layer. The basic mechanism of organic EL involves the injection, transport, and recombination of carriers as well as exciton formation for emitting light. When an external voltage is applied across the organic EL device, electrons and holes are injected from the cathode and the anode, respectively. Electrons will be injected from the cathode into a LUMO (lowest unoccupied molecular orbital) and holes will be injected from the anode into a HOMO (highest occupied molecular orbital). Subsequently, the electrons recombine with holes in the light emitting layer to form excitons, which then deactivate to emit light. When luminescent molecules absorb energy to achieve an excited state, the exciton may either be in a singlet state or a triplet state, depending on how the spins of the electrons and holes have been combined. It is well known that the excitons formed under electrical excitation typically include 25% singlet excitons and 75% triplet excitons. In the fluorescence materials, however, the electrically generated energy in the 75% triplet excitons will be dissipated as heat for decay from the triplet state is spin forbidden. Therefore, a fluorescent electroluminescence device has only 25% internal quantum efficiency, which leads to the theoretically highest external quantum efficiency (EQE) of only 5% due to only ˜20% of the light out-coupling efficiency of the device.
In 2012, a new type of fluorescent organic EL device was developed by Adachi and coworkers. The new organic EL device incorporated the mechanism of thermally activated delayed fluorescence (TADF), which was a promising way to obtain a high percentage of singlet exciton formation by converting spin-forbidden triplet excitons up to the singlet level through the mechanism of reverse intersystem crossing (RISC). However, there still exists a need for the compound with TADF property to improve the performance of the organic EL devices.